We wish to study the phosphorylation-dephosphorylation of myosin light chain during the contraction-relaxation cycle of the fast, extensor digitorum longus, and slow, soleus, muscles of the rat. These muscles will be dissected from the animal, incubated in physiological salt solution at 37C, stimulated and frozen with an automatic apparatus at various stages of the contraction relaxation cycle. The phosphate content of light chain will be determined either as moles [32P] phosphate per mole of light chain in 32P-labeled muscles or as percentage phosphorylated light chain of the total light chain, measured by densitometry after separating the phospho and dephospho forms of light chain with two-dimensional gel electrophoresis. The aim of this study is to determine the relationship between light chain phosphorylation and contraction in rat muscle and compare it with that determined in chicken and frog muscles previously. Turtle hearts will be perfused with 32P at room temperature, then transferred into the cold room to slow down the heart beat, to about 6 revolutions per minute. The hearts will be frozen either in the systolic or diastolic state manually and the [32P] phosphate content of the myosin light chain will be determined. This study aims to find out whether any relationship exists between light chain phosphorylation and heart cycle.